Over-current protection circuits can be categorized into systems based on voltage comparison and current comparison. Over-current protection circuits based on voltage comparison have a slow response speed or a relatively large power consumption. To address this issue protection circuits based on current comparison have been proposed.
For instance, an over-current protection circuit has been described in the publication titled “A monolithic current-mode buck converter with advanced control and protection circuits”. IEEE Transactions on Power Electronics, pp. 1836-1846, Vol. 22, No. 5, September 2007.
The protection circuit has a current sensing circuit and a comparator described with respect to FIGS. 7 and 9(b) of the above-mentioned application. The current sensing circuit is used to generate a current Iz that is inversely proportional to the drain current of a power switch M1 directly connected to the switching node of a buck converter. The power switch M1 is connected to a CCII current conveyor via a second power switch M2 of same type as M1 but having different aspect ratio. The CCII current conveyor regulates the drain voltage of M2 to be the same as that of M1. Hence, the biasing condition of M2 is almost the same as that of M1, and the current Iz is almost inversely proportional to the drain current of M1. The comparator circuit then compares Iz against a reference current. If Iz is larger than the reference current, the comparator will output a logic high signal indicating an over-current condition.
The comparator circuit described above is fast and consumes little energy, however such a circuit is not suitable for negative over-current protection. In particular, such a circuit is not suitable for protecting an inverting boost converter. The current circuit is designed to operate with a positive power domain (0V˜6V); whereas for an inverting boost converter, the current sensing circuit would need to operate with a negative domain (−6V˜0V). In addition, in an inverting boost converter, the voltage at the switching node can swing between relatively large negative and positive value, for example −6V to +6V, therefore applying the sensing circuit of the prior art to an inverting boost converter would stress the current conveyor.
In addition, the comparator circuit of the prior art is limited in that one can only reduce the comparator delay by increasing the sensing current and the quiescent current of the current conveyor circuit; which increases power consumption.